1. Field of the Invention
The present invention relates to an electroluminescent display device, and more particularly, to an organic electroluminescent display device including thin film transistors with amorphous silicon as active layers and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving shift of threshold voltage and decreasing current in an organic electroluminescent display device.
2. Discussion of the Related Art
In general, an organic electroluminescent display device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons and the holes to generate an exciton, and by transiting the exciton from an excited state to a ground state. Since the organic electroluminescent display device does not require an additional light source due to its self-luminescence property, the organic electroluminescent display device has a small size and is light in weight, as compared to a liquid crystal display device. The organic electroluminescent display device also has a low power consumption, high brightness, and a short response time. Thus, the organic electroluminescent display device is used in many consumer electronic applications, such as cellular phones, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm PCs. In addition, the organic electroluminescent display device can reduce manufacturing costs because of its simple manufacturing processes.
Organic electroluminescent display devices may be categorized into a passive matrix-type and an active matrix-type depending upon the method used to drive the device. Passive matrix-type organic electroluminescent display devices have a simple structure and are fabricated through a simple manufacturing process. However, the passive matrix-type organic electroluminescent display devices have high power consumption, thereby preventing use in large area displays. Furthermore, in passive matrix organic electroluminescent display devices, an aperture ratio decreases due to the increased number of electrical lines. Thus, the passive matrix-type organic electroluminescent display devices are commonly used as small-sized display devices. On the other hand, active matrix-type organic electroluminescent display (AMOELD) devices are commonly used as large-sized display devices since they have a high luminous efficiency, and provide high definition images.
FIG. 1 is a cross-sectional view of an active matrix-type organic electro-luminescent display (AMOELD) device according to the related art. In FIG. 1, the AMOELD device 10 includes a first substrate 12 and a second substrate 28, which are spaced apart and face each other. The first substrate 12 may be transparent and flexible. A plurality of thin film transistors T and a plurality of first electrodes 16 are formed on an inner surface of the first substrate 12, in which each of first electrodes 16 is connected to the respective thin film transistor T. Organic layers 18 are formed on the first electrodes 16 and the thin film transistors T, and a second electrode 20 is formed on the organic layers 18. The organic layers 18 emit light of three colors: red (R), green (G), and blue (B) within a pixel region P, and are generally formed by patterning an organic material that emits one of red, green and blue.
A desiccant 22 is formed on an inner surface of the second substrate 28 to remove any external moisture and air that may permeate into the space between the first and second substrates 12 and 28. The inner surface of the second substrate 28 is patterned to form a groove, and the desiccant 22 is disposed within the groove and is fastened with a tape 25.
A sealant 26 is formed between the first and second substrates 12 and 28 to attach the first and second substrates 12 and 28, and surrounds elements, such as the thin film transistors T, the first electrodes 16, the organic layers 18, and the second electrodes 20. The sealant 26 forms an airtight space to protect the elements from the external moisture and air. The first electrode 16 functions as an anode electrode and is transparent. Thus, this AMOELD device has a bottom emission type, in which light is emitted through the first electrode 16.
FIG. 2 is an equivalent circuit for a pixel of an organic electroluminescent display (OELD) device according to the related art. As shown in FIG. 2, a gate line 13 is formed along one direction of a substrate 12 and a data line 15 crosses the gate line 13. A switching element TS is formed at a crossing point of the gate line 13 and the data line 15, and a driving element TD is electrically connected to the switching element TS.
A storage capacitor CST is disposed between a source electrode of the driving element TD and a gate electrode of the driving element TD, and a drain electrode of the driving element TD is connected to a first electrode of an organic electroluminescent diode E. A second electrode of the organic electroluminescent diode E is connected to a power supply line 21, which supplies a power source VDD.
The OELD device having the above structure can be driven as follows.
First, when a gate ON signal is applied to a gate electrode of the switching element TS, a current signal flowing through the data line 49 is changed into a voltage signal through the switching element TS and is applied to the gate electrode of the driving element TD. Then, the driving element TD turns on, and thus the gray scale is realized by determining levels of the current flowing through the organic electroluminescent diode E.
At this time, because signals stored in the storage capacitor CST maintain the signal of the gate electrode of the driving element TD, the level of the current flowing through the organic electroluminescent diode E is kept constant until a next signal is applied even if the switching element TS turns off. The switching element TS and the driving element TD may be an amorphous silicon thin film transistor or a polycrystalline silicon thin film transistor. The amorphous silicon thin film transistor is more simply manufactured as compared with the polycrystalline silicon thin film transistor.
FIG. 3 is a cross-sectional view illustrating a driving element having an amorphous silicon thin film transistor for an OELD device according to the related art.
In FIG. 3, a gate electrode 34 of the driving element is formed on a substrate 30. A gate insulating layer 38 is formed on the entire surface of the substrate 30 having the gate electrode 34 thereon. A semiconductor layer 58 is formed on the gate insulating layer 38. The semiconductor layer 58 includes an active layer 58a and an ohmic contact layer 58b that are sequentially deposited. A part of the active layer 58a functions as a channel CH of the driving element. Source and drain electrodes 52 and 54 are formed on the semiconductor layer 58. The source and drain electrodes 52 and 54 contact the ohmic contact layer 58b and are spaced apart from each other. A first passivation layer 60 is formed on the entire surface of the substrate 30 including the source and drain electrodes 52 and 54 thereon, and a ground line 62 is formed on the first passivation layer 60 to earth the source electrode 52. A second passivation layer 64 is formed on the entire surface of the substrate 30 including the ground line 62, and a first electrode 66 of an organic electroluminescent diode is formed on the second passivation layer 64 in a pixel region. The first electrode 66 contacts the drain electrode 54. Although not shown in the figure, an organic light-emitting layer and a second electrode are formed on the first electrode 66. Current uniformly flows through the organic light-emitting layer while a signal on the gate electrode of the driving element is kept constant until the next signal is applied. However, the current may decrease due to long term degradation of the driving element.
FIG. 4A is a graph illustrating voltage versus current (V-I) characteristics of a driving element and an organic electroluminescent diode according to the related art. FIG. 4B is a graph illustrating time versus variation of current through the organic electroluminescent diode in the related art. As shown in FIG. 4A, curves M1 and M2 illustrate voltage versus current characteristics of the driving element, and curves N1 and N2 illustrate voltage versus current characteristics of the organic electroluminescent diode in the relate art. The curves M1 and N1 correspond to the case that time t is t0, that is, a gate ON signal V0 is applied on the gate electrode of the driving element. The curves M2 and N2 correspond to the case that time t is t1, that is, the gate ON signal on the gate electrode of the driving element becomes V1 due to the long term degradation of the driving element, wherein V1 is lower than V0. Thus, the current I flowing through the organic electroluminescent diode decreases from I0 at t=t0 to I1 at t=t1. Accordingly, as shown in FIG. 4B, the current I decreases until the half-life of the organic electroluminescent diode.
FIG. 5 illustrates a surface potential between an active layer and a gate insulating layer of a driving element of the related art, i.e., along line A-A′ of FIG. 3, in a saturation region when the driving element turns ON. As shown in FIG. 5, the surface potential concentrates in a region corresponding to an edge portion D of the drain electrode 54 of FIG. 3. Therefore, in the region corresponding to the edge portion D of the drain electrode 54, defects are formed due to bond-breaking of a Si—Si weak bond in the active layer 58a of FIG. 3 and charge-trapping. Accordingly, a threshold voltage is shifted to decrease current flowing through the organic electroluminescent diode. Brightness and life span of the OELD are reduced due to the decreased current and the defects.